1. Field of the Invention
Embodiments of the present invention generally relate to the fabrication of solar cells and particularly to the surface passivation of crystalline silicon solar cells.
2. Description of the Related Art
Solar cells are photovoltaic devices that convert sunlight directly into electrical power. The most common solar cell material is silicon (Si), which is in the form of single or multi-crystalline wafers. Because the cost of electricity generated using silicon-based solar cells is higher than the cost of electricity generated by traditional methods, there has been an effort to reduce the cost of manufacturing solar cells that does not adversely affect the overall efficiency of the solar cell.
FIG. 1 schematically depicts a cross-sectional view of a standard silicon solar cell 100 fabricated from a single crystal silicon wafer 110. The wafer 110 includes a base region 101, which is typically composed of p-type silicon, an emitter region 102, which is typically composed of n-type silicon, a p-n junction region 103, and a dielectric layer 104. P-n junction region 103 is disposed between base region 101 and emitter region 102 of the solar cell, and is the region in which electron-hole pairs are generated when solar cell 100 is illuminated by incident photons. Dielectric layer 104 acts as an anti-reflective coating (ARC) layer for solar cell 100 as well as a passivation layer for the surface 105 of emitter region 102.
When light falls on the solar cell, energy from the incident photons generates electron-hole pairs on both sides of p-n junction region 103. Electrons diffuse across the p-n junction to a lower energy level and holes diffuse in the opposite direction, creating a negative charge on the emitter and a corresponding positive charge build-up in the base. When an electrical circuit is made between the emitter and the base, a current will flow and electricity is produced by solar cell 100. The efficiency at which solar cell 100 converts incident energy into electrical energy is affected by a number of factors, including the recombination rate of electrons and holes in solar cell 100 and the fraction of incident light that is reflected off of solar cell 100.
Recombination occurs when electrons and holes, which are moving in opposite directions in solar cell 100, combine with each other. Each time an electron-hole pair recombines in solar cell 100, charge carriers are eliminated, thereby reducing the efficiency of solar cell 100. Recombination may occur in the bulk silicon of wafer 110 or on either surface 105, 106 of wafer 110. In the bulk, recombination is a function of the number of defects in the bulk silicon. On the surfaces 105, 106 of wafer 110, recombination is a function of how many dangling bonds, i.e., unterminated chemical bonds, are present on surfaces 105, 106. Dangling bonds are found on surfaces 105, 106 because the silicon lattice of wafer 110 ends at these surfaces. These unterminated chemical bonds act as defect traps, which are in the energy band gap of silicon, and therefore are sites for recombination of electron-hole pairs.
Thorough passivation of the surface of a solar cell greatly improves the efficiency of the solar cell by reducing surface recombination. As used herein, “passivation” is defined as the chemical termination of dangling bonds present on the surface of a silicon lattice. In order to passivate a surface of solar cell 100, such as surface 105, a dielectric layer 104 is typically formed thereon, thereby reducing the number of dangling bonds present on surface 105 by 3 or 4 orders of magnitude. For solar cell applications, dielectric layer 104 is generally a silicon nitride (Si3N4, also abbreviated SiN) layer, and the majority of dangling bonds are terminated with silicon (Si) or nitrogen (N) atoms. But because silicon nitride (SiN) is an amorphous material, a perfect match-up between the silicon lattice of emitter region 102 and the amorphous structure of dielectric layer 104 cannot occur. Hence, the number dangling bonds remaining on surface 105 after the formation of dielectric layer 104 is still enough to significantly reduce the efficiency of solar cell 100, requiring additional passivation of surface 105, such as hydrogen passivation. In the case of multi-crystalline silicon solar cells, hydrogen also helps to passivate the defect centers on the grain boundary.
When dielectric layer 104 is a silicon nitride (SiN) layer, hydrogen passivation of surface 105 is performed by the incorporation of an optimal concentration of hydrogen (H) atoms in the bulk of dielectric layer 104. The optimal hydrogen concentration is a function of numerous factors, including the film properties and the method of depositing dielectric layer 104, but ranges between about 5 atomic % and 20 atomic %. After deposition of dielectric layer 104, solar cell substrates undergo a high-temperature anneal process, sometimes referred to as a “firing process,” which forms the metal contacts with the cells. During the firing process, hydrogen atoms present in dielectric layer 104 diffuse from dielectric layer 104 to surface 105, and grain boundaries if multi-crystalline silicon, and terminate many of the remaining dangling bonds present there. It is known in the art that having the optimal concentration of hydrogen atoms in the bulk of dielectric layer 104 is a key factor for enabling an effective hydrogen passivation process. For example, it is estimated that poor hydrogen passivation of surface 105 may reduce the potential efficiency of a solar cell from approximately 14-15% efficiency down to 12-13% efficiency or less.
In addition to surface passivation, the efficiency of solar cell 100 may be enhanced with an ARC layer. When light passes from one medium to another, for example from air to glass, or from glass to silicon, some of the light may reflect off of the interface between the two media, even when the incident light is normal to the interface. The fraction of light reflected is a function of the difference in refractive index between the two media, wherein a greater difference in refractive index results in a higher fraction of light being reflected from the interface. An ARC layer disposed between the two media and having a refractive index whose value is between the refractive indices of the two media is known to reduce the fraction of light reflected. Hence, the presence of an ARC layer on a light-receiving surface of solar cell 100, such as dielectric layer 104 on surface 105, reduces the fraction of incident radiation reflected off of solar cell 100 and which, therefore, cannot not be used to generate electrical energy. Dielectric layer 104 is most effective as an ARC layer when the index of refraction of dielectric layer 104 is equal to the square root of the product of the indices of the two media forming the interface that causes the reflection, in this case glass and silicon. The refractive indices of glass and silicon are about 1.4 and 3.4, respectively, so an ideal ARC layer for a solar cell should have a refractive index of about 2.1. Because the refractive index of SiN is tunable between about 1.8 to 3.0 by modulating process parameters of the SiN deposition process, SiN is suitable as an ARC layer for solar cells.
Because SiN can be used to terminate dangling bonds on a silicon surface, and because it has a refractive index that can be tuned to a desired value, SiN is widely used as a combination ARC layer and passivation layer for solar cell applications. However, issues with SiN deposition on solar cell substrates include low throughput and poor film property uniformity. Throughput, i.e., the rate at which solar cell substrates are, processed, directly affects the cost of processing solar cell substrates. Low throughput of a SiN deposition system ultimately increases solar cell cost. Film property non-uniformity, both wafer-to-wafer, i.e., variation between substrates, and within wafer, i.e., film variation across an individual substrate, may affect the performance of solar cells. Namely, sub-optimal uniformity of properties of the SiN film reduces the efficiency of the solar cell.